Alison Raby

Ground motion characteristics and shaking damage of the 11th March 2011 Mw9.0 Great East Japan earthquake

A catastrophic Mw9.0 earthquake and subsequent giant tsunami struck the Tōhoku and Kanto regions of Japan on 11th March 2011, causing tremendous casualties, massive damage to structures and infrastructure, and huge economic loss. This event has revealed weakness and vulnerability of urban cities and modern society in Japan, which were thought to be one of the most earthquake-prepared nations in the world. Nevertheless, recorded ground motion data from this event offer invaluable information and opportunity; their unique features include very strong short-period spectral content, long duration, and effects due to local asperities as well as direction of rupture/wave propagation. Aiming at gaining useful experience from this tragic event, Earthquake Engineering Field Investigation Team (EEFIT) organised and dispatched a team to the Tōhoku region of Japan. During the EEFIT mission, ground shaking damage surveys were conducted in Sendai, Shirakawa, and Sukagawa, where the Japan Meteorological Agency intensity of 6+ was observed and instrumentally recorded ground motion data were available. The damage survey results identify the key factors for severe shaking damage, such as insufficient lateral reinforcement and detailing in structural columns from structural capacity viewpoint and rich spectral content of ground shaking in the intermediate vibration period range from seismic demand viewpoint. Importantly, inclusion of several ground motion parameters, such as nonlinear structural response, in shaking damage surveys, can improve the correlation of observed ground motion with shaking damage and therefore enhance existing indicators of potential damage.

Pure and aerated water entry of a flat plate

This paper presents an experimental and numerical investigation of the entry of a rigid square flat plate into pure and aerated water. Attention is focused on the measurement and calculation of the slamming loads on the plate. The experimental study was carried out in the ocean basin at Plymouth University’s COAST laboratory. The present numerical approach extends a two-dimensional hydro-code to compute three-dimensional hydrodynamic impact problems. The impact loads on the structure computed by the numerical model compare well with laboratory measurements. It is revealed that the impact loading consists of distinctive features including (1) shock loading with a high pressure peak, (2) fluid expansion loading associated with very low sub-atmospheric pressure close to the saturated vapour pressure, and (3) less severe secondary reloading with super-atmospheric pressure. It is also disclosed that aeration introduced into water can effectively reduce local pressures and total forces on the flat plate. The peak impact loading on the plate can be reduced by half or even more with 1.6% aeration in water. At the same time, the lifespan of shock loading is prolonged by aeration, and the variation of impulse is less sensitive to the change of aeration than the peak loading.

Numerical and physical modeling of extreme waves at Wave Hub

ABSTRACT Ransley, E., Hann, M., Greaves, D., Raby, A. and Simmonds, D., 2013. Numerical and physical modelling of extreme waves at Wave Hub With a history of international failures, the survivability of coupled systems of wave energy devices and their moorings, particularly those to be installed at development sites like Wave Hub, is surrounded by uncertainty. Potential design solutions require a better understanding of the hydrodynamics and structural loading experienced during extreme events, like rogue wave impact, in order to mitigate the risk of device and mooring failure. Rogue waves are waves with amplitudes far greater than those expected, given the surrounding sea conditions. Intense study into these events stems from their potential for catastrophic impact on ocean engineering structures. However, little is known about their physical origins and, currently, there is no consensus on their definition or explanation of the mechanism which drives them. This paper concerns the numerical modeling and experimental validation of extreme rogue wave examples at the Wave Hub site. Using hindcast data, the 100 year extreme wave at the Wave Hub site is determined. This extreme wave is replicated in Plymouth Universitys new COAST Lab using a NewWave, dispersive focusing input. To simulate and analyse these events, we duplicate these conditions in a numerical wave tank (NWT), solving the fully nonlinear Navier-Stokes equations, with a free surface, using the Volume of Fluid (VoF) method and open source CFD library OpenFOAM®. The comparison shows that the CFD software is capable of simulating focused waves similar to those produced in the physical tank but tends to overestimate the crest heights. It is also noted that nonlinear effects are important when considering the shape and location of focused wave events.

Finite Element Modelling and Limit Analysis of Fastnet Lighthouse Under Impulsive Ocean Waves

Being exposed to strong ocean waves for more than a century, the Fastnet lighthouse is assessed for its structural response to the intense lateral loading. The Finite Element Method (FEM) was implemented for the structural analysis using the commercial software Abaqus. The lighthouse is built with large and meticulously dovetailed granite blocks which make it a very unique structural system. Three different finite element model configurations were tested, modelling the lighthouse as continuous homogeneous (elastic and nonlinear), and as discontinuous with contact interfaces between each course of blocks allowing uplift and sliding. The applicability and efficacy of these approaches is discussed. The impact load of the wave was applied as a time-history sequence, assuming that the wave breaks just in front of the structure surface corresponding to the least favourable scenario. Different intensities and heights were considered for the impact load. Finally, the FEM results are also compared with the results of the limit analysis method which calculates the minimum intensity of lateral static load that is necessary for causing uplift and overturning of rigid bodies. This comparison demonstrates the usefulness of the limit analysis method as a tool for quick preliminary assessment of the lateral load bearing capacity of this particular structural typology. This work is part of the STORMLAMP project (STructural behaviour Of Rock Mounted Lighthouses At the Mercy of imPulsive waves) funded by the UK Engineering and Physical Sciences Research Council, which is gratefully acknowledged.

Exploring the Potential for Multivariate Fragility Representations to Alter Flood Risk Estimates: Potential for Multivariate Fragility Representations to Alter Flood Risk Estimates

In flood risk analysis, limitations in the multivariate statistical models adopted to model the hydraulic load have restricted the probability of a defense suffering structural failure to be expressed conditionally on a single hydraulic loading variable. This is an issue at the coastal level where multiple loadings act on defenses with the exact combination of loadings dictating their failure probabilities. Recently, a methodology containing a multivariate statistical model with the flexibility to robustly capture the dependence structure between the individual loadings was used to derive extreme nearshore loading conditions. Its adoption will permit the incorporation of more precise representations of a structures vulnerability in future analyses. In this article, a fragility representation of a shingle beach, where the failure probability is expressed over a three-dimensional loading parameter space-water level, wave height, and period-is derived at two localities. Within the approach, a Gaussian copula is used to capture any dependencies between the simplified geometric parameters of a beachs shape. Beach profiles are simulated from the copula and the failure probability, given the hydraulic load, determined by the reformulated Bradbury barrier inertia parameter model. At one site, substantial differences in the annual failure probability distribution are observed between the new and existing approaches. At the other, the beach only becomes vulnerable after a significant reduction of the crest height with its mean annual failure probability close to that presently predicted. It is concluded that further application of multivariate approaches is likely to yield more effective flood risk management.

Tsunami design procedures for engineered buildings: a critical review

Tsunamis have the potential to cause enormous loss of life and socio-economic impacts on coastal communities. Central to tsunami risk mitigation is the protection of critical infrastructure and evacuation-designated buildings, which are often necessarily located within tsunami inundation zones. As such, these must be designed to withstand and remain fully or partially operational after a tsunami. Guidance documents for tsunami design of buildings exist in the USA and Japan, including the recent release of the US ASCE 7 chapter 6 on tsunami loads and effects. This paper outlines the key engineering principles of tsunami design of buildings, summarises and compares how these principles are addressed by US and Japanese standards, and outlines considerations not yet covered.

Experimental Investigation of Elasticity Effects on Slamming

Elasticity effects on slamming was investigated by dropping a flat plate onto the water surface. The experimental work was carried out in the ocean basin at Plymouth University’s COAST Laboratory. The ocean basin is 35 m long by 15.5 m wide and has an adjustable floor that allows operation at different water depths up to 3 m. The falling block includes a rigid impact plate connected to two driver plates and its total mass can be varied between 32 kg to 52 kg. A spring system was used to form elastic plates and elasticity of the plate could be changed using different spring stiffness. The impact plate has dimension of 0.25 m long, 0.25 m wide and 0.012 m high. Pressures under the impact plate were measured during impact by several pressure transducers (FGP Sensors XPM10 having measurement range of up to 100 bar) installed on the impact plate. Result shows that elasticity of the impact plate has a significant effect on impact loads, with impact loads being considerably reduced by decreasing the plate-stiffness.